1. Field of the Invention
The present invention relates to new polyester polyols based on selected starting materials and to their use in combination with certain lacquer polyiso-cyanates as binders or binder components in two-component polyurethane lacquers for the coating of molded plastics parts.
2. Description of the Prior Art
It has long been known to coat relatively inflexible substrates such as metal or wood with two-component polyurethane lacquers based on hydroxyl-containing polyesters, polyethers or polyacrylates and organic polyisocyanates. The resulting lacquer coats are distinguished in particular by their excellent hardness, very good adherence and unsurpassed weather resistance. The chemical basis of these lacquers and coatings has been described inter alia in "Lackkunstharze" Hans Wagner/Hans Friederich Sarx, Carl Hanser Verlag, Munich, pages 153 to 173, 1971.
The two-component polyurethane lacquers known in the art, however, frequently result in highly cross-linked lacquer coats which in many cases have insufficient elasticity to meet the requirements of coatings on flexible substrates. It is particularly in the construction of motor vehicles that flexible plastics parts are being used to an increasing extent for the purpose of improved safety. Thus, for example, in some countries, motor vehicles must be equipped with bumpers capable of absorbing an impact against a solid barrier at a certain velocity (e.g. 5 MPH) without damage and at the same time functional parts such as headlights, flashing lights, doors and hoods must be unimpaired in their function. Due to these and other more far-reaching requirements, the metal bumpers hitherto used in many countries have been abandoned in the construction of passenger cars and replaced by complete front and rear parts made of highly flexible materials capable of meeting the requirements that the bumper should recoil after impact and be left undamaged (prevention of damage in minor accidents).
The highly flexible plastics materials which have proved to be suitable for this purpose include inter alia semi-rigid elastomeric polyurethanes (e.g. Bayflex types from BAYER AG, Leverkusen) produced from two-component polyurethane-forming mixtures by the reaction injection molding technique in closed molds, optionally with foaming; thermoplastic polyurethanes (e.g. Desmopan types from BAYER AG or Texin types from Mobay Chemical Corporation, Pittsburgh, USA) which are worked up by the injection molding technique, as well as various types of rubbers.
Plastics parts based on these chemical raw materials, which have for some years now belonged to the state of the art in the construction of motor vehicles, are relatively large and therefore to a significant extent determine the appearance of the vehicle. For these reasons, lacquering of the unfinished parts is necessary. To this is added the fact that the surface of the plastics materials are degraded on exposure to weathering and must therefore be protected against weathering influences. Elastic lacquer coats are, however, also necessary for less elastic plastics parts to prevent mechanical damage to the parts. Thus, for example, rigid but tough thermoplasts must be lacquered with highly elastic, extremely resistant lacquers in order to prevent tearing of the lacquer film as a result of mechanical damage or other external influences and propagation of the tears through the solid plastics material. The lacquer coats, and especially the top lacquer coat on such parts must therefore meet much higher standards than a conventional lacquer coat.
It was therefore an object of the present invention to provide a new two-component polyurethane lacquer system which would satisfy these special requirements and in particular would be optimally suitable for the lacquering of elastic plastics parts. The new lacquer system should in particular satisfy the following requirements:
1. Low temperature elasticity: The film must not tear under the above-mentioned velocities of impact even at -25.degree. C. The mechanical properties of the plastics material should as much as possible be left unimpaired by the lacquer coating.
2. Good gloss retention: The lacquer film on the plastics parts should be as durable as possible. It must not be degraded more rapidly under weathering conditions than the film on the car body.
3. No subsequent increase in brittleness on exposure to weathering: Even after prolonged weathering the film must not become more brittle, i.e. the low temperature elasticity should not be reduced in the course of weathering.
4. Low drying temperature: The heat resistance of such large plastics parts is limited. The drying temperatures and times required should be low not only for this reason but also for the sake of saving energy.
5. Repairability: The drying conditions of such a system should enable not only the initial lacquering but also repair lacquering to be carried out at 80.degree. C. or room temperature.
Although highly flexible lacquers which are perfectly satisfactory in their elasticity and cold flexibility can be produced from the known two-component systems of the state of the art, this is due mainly to the use, as the main polyol component, of certain polyhydroxyl polyesters synthesized predominantly from aliphatic dicarboxylic acids and aliphatic diols and having a hydroxyl functionality which may be slightly, but not significantly above 2. The disadvantage of two-component polyurethane lacquers based on such polyester diols and the conventional lacquer polyisocyanates is that the lacquer coatings obtained are much too soft, i.e. they have insufficient surface hardness. The weather resistance particularly, and the resistance to chalking and hence the gloss retention of coatings produced from such lacquer systems are completely inadequate.
The so-called one-pack coating compounds described in the patent literature (e.g. DE-OS No. 2 407 495), which contain inter alia a urethanized polyester polyol and, as cross-linking component, a melamine formaldehyde resin, for example, and which are particularly suitable for lacquer coating rubber-like, springy substrates (such as foams of polyurethanes or natural or synthetic rubber) are also not optimally suited for lacquering parts of motor vehicles for which the above-mentioned requirements are applicable. One particular disadvantage is that hardening of the lacquers is required to be carried out at elevated, temperatures, with the result that the molded plastics parts, which frequently only have a low heat resistance, undergo deformation during stoving of the lacquer.
It has not surprisingly been found that these problems may be solved by using the polyester polyols according to the invention described below as a portion or all of the polyhydroxyl component in two-component polyurethane lacquers.